(a) Field of the Invention
The present invention relates to a heterodyne receiver including a frequency converter for low noise and image frequency repression being applied to a heterodyne communication system.
(b) Description of the Related Art
FIG. 1 shows the image frequency problem of a frequency converter.
As shown in FIG. 1, a desired signal W1 and an image frequency are separated by intermediate frequency WIF from a local oscillating frequency LO in both directions. Both signals are then converted by the frequency converter to the same intermediate frequency.
In particular, distortion of a desired signal increases during frequency conversion when the image signal is large. As a result, an additional function to suppress the image frequency is required.
FIG. 2 shows a brief construction of a heterodyne receiver according to a conventional method.
As shown in FIG. 2, the heterodyne receiver according to a conventional method has a filter located between a low noise amplifier and a frequency converter for suppressing the image frequency.
The filter for suppressing the image frequency in the heterodyne receiver can be implemented as a manual type filter for external usage or as a filter having a Hartely or Weaver construction.
However, the embodiment of above with Hartely architecture or Weaver architecture has the problem of being complicated, so it is typical for a filter for suppressing an image frequency being added between the low noise amplifier and the frequency converter.
FIG. 3 to FIG. 5 show circuits of low noise amplifiers including filters for suppressing an image frequency according to a first, second, and third example of the conventional methods.
First, the low noise amplifier including the filter for suppressing an image frequency according to the first example of the conventional method includes a filter circuit for image suppressing composed of a first inductor L1 and a first capacitor C1 functioning as a filter and an amplifier circuit composed of transistors M1 and M2 and a second inductor L2.
Impedance
<PSTYLE ∈ DENT = 0LSPACE = 130 > |Zf|at a node X is reduced by a series resonance between the first inductor and the first capacitor, and thus a signal current which is applied to the transistor M2 is reduced. As a result, an image frequency gain, Vout/Vin is reduced.
Meanwhile, FIG. 3b shows that the low noise amplifier including a filter for suppressing an image frequency according to the first example has a property for suppressing the image frequency.
Next, the low noise amplifier including a filter for suppressing an image frequency according to the second example of the conventional method has a similar construction to the first example except for the circuit for suppressing the image frequency.
That is, impedance at the node X is reduced by a parasitic capacitance Cx when an Lx is absent as shown in FIG. 4a. Thus, the gain of a CASCODE composed of the transistors M1 and M2 is reduced.
Further, the Cx increases influence of the noise from the transistor M2 and decreases influence of the signal current from the transistor M1 to the transistor M2. Thus, the property of a noise figure of the low noise amplifier worsens.
Thus, the low noise amplifier including a filter for suppressing an image frequency according to the second example has a frequency property of the noise figure as shown in FIG. 4b. From the frequency property, it can be known that the noise figure can be improved by parallel resonance between Cx and Lx at the signal frequency fsig.
The low noise amplifier including a filter for suppressing an image frequency according to the third example is to implement a third filter by properly combining the constructions of the first example and the second example.
The low noise amplifier including a filter for suppressing an image frequency includes a first inductor L1, a capacitor C2, and a variable capacitor C1 as shown in FIG. 5a. The low noise amplifier can modify the image frequency which is desired to be suppressed, by changing the capacitance of C1.
The impedance property of the third filter is shown in FIG. 5b. Impedance
<PSTYLE ∈ DENT = 0LSPACE = 130 > |Zf|at the node X is reduced at the image frequency fimg, and is increased at the signal frequency fsig.
As shown in FIG. 5c, for the input-output property of the amplifier, the Vout/Vin at the image frequency has ‘V’ type narrowband properties. Correct control of the filter for series resonance at an image frequency is required to suppress the image frequency. On the other hand, the flat input-output property of a proper wideband is provided at around the signal frequency, so correct control for parallel resonance is not required.
Thus, the third filter suppresses the image frequency and offsets the influence of the parasitic capacitance at the node X. The noise property is therefore improved.
FIG. 6 shows a circuit of an active frequency converter according to a first example of the conventional method, and FIG. 7 shows a circuit of an active frequency converter according to a second example of the conventional method.
The active frequency converters shown in FIG. 6 and FIG. 7 provide a conversion gain by using a MOSFET, and are examples without filter circuits.
First, according to the active frequency converter shown in FIG. 6, a single input RF voltage signal is converted to a current signal by the transistor M1 in a drive 11. An LO signal applied from a local oscillator LO is multiplied to the converted current signal by the transistors M2 and M3 with switching functions.
Next, according to the active frequency converter shown in FIG. 7, the differential RF voltage signal is converted to a current signal by the transistors M1 and M2. An LO signal applied from a local oscillator LO is multiplied to the converted current signal by the transistors M3-M4 and M5-M6 with switching functions.
FIG. 6 shows a single balanced mixer circuit wherein the single input signal is inputted to the first drive, and FIG. 7 shows a double balanced mixer circuit wherein the differential input signal is inputted.
FIG. 8 shows the folding of RF and image noise into the IF band with a conventional frequency converter.
As shown in FIG. 8, a frequency component which is inputted to an input port X is composed of a signal component at a signal frequency band and a thermal noise due to a load resistor Rs of the input port at the signal frequency band and the image frequency band.
The signal component, the thermal noise at the signal frequency band, and the thermal noise at the image frequency are converted to signals at the intermediate frequency band in the process of frequency conversion. Thus, a signal-to-noise ratio at an output port Y becomes ½ the signal-to-noise ratio at the input port X, when the conversion gain at the signal frequency band is the same as the conversion gain at the image frequency band.
Thus, the noise figure property of the frequency converter becomes worse by a factor of two (3 dB) when the gain of the frequency converter is 1. That is, the noise figure property can be improved only when the gain of the conversion at the image frequency band is lower than the gain of the conversion at the signal frequency band. Otherwise, the noise figure property worsens.
Meanwhile, most communication systems require image frequency suppression of more than 60˜70 dB to reject an image. However, the conventional method using the filter in the low noise amplifier cannot meet the system requirement. Thus, a device for suppressing an image is further required, apart from the low noise frequency amplifier.